The invention relates to superconducting magnetic coils and methods for manufacturing them.
As is known in the art, the most spectacular property of a superconductor is the disappearance of its electrical resistance when it is cooled below a critical temperature T.sub.c. Another important property is the destruction of superconductivity by the application of a magnetic field equal to or greater than a critical field H.sub.c. The value of H.sub.c, for a given superconductor, is a function of the temperature, given approximately by EQU H.sub.c =H.sub.o (1-T.sub.c.sup.2)
where H.sub.o, the critical field at 0.degree. K., is, in general, different for different superconductors. For applied magnetic fields less than H.sub.c, the flux is excluded from the bulk of the superconducting sample, penetrating only to a small depth, known as the penetration depth, into the surface of the superconductor.
The existence of a critical field implies the existence of a critical transport electrical current, referred to more simply as the critical current (I.sub.c) of the superconductor. The critical current is the current which establishes the point at which the material loses its superconductivity properties and reverts back to its normally conducting state.
Superconducting materials are generally classified as either low or high temperature superconductors operating below or at 4.2.degree. K. and below or at 108.degree. K., respectively. High temperature superconductors (HTS), such as those made from ceramic or metallic oxides are anisotropic, meaning that they generally conduct better in one direction than another. Moreover, it has been observed that, due to this anisotropic characteristic, the critical current varies as a function of the orientation of the magnetic field with respect to the crystallographic axes of the superconducting material. High temperature oxide superconductors include general Cu-O-based ceramic superconductors, members of the rare-earth-copper-oxide family (YBCO), the thallium-barium-calcium-copper-oxide family (TBCCO), the mercury-barium-calcium-copper-oxide family (HgBCCO), and BSCCO compounds containing stoichiometric amounts of lead (ie.,(Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10).
High temperature superconductors may be used to fabricate superconducting magnetic coils such as solenoids, racetrack magnets, multipole magnets, etc., in which the superconductor is wound into the shape of a coil. When the temperature of the coil is sufficiently low that the conductor can exist in a superconducting state, the current carrying capacity as well as the magnitude of the magnetic field generated by the coil is significantly increased.
In fabricating such superconducting magnetic coils, the superconductor may be formed in the shape of a thin tape which allows the conductor to be bent around relatively small diameters and allows the winding density of the coil to be increased. The thin tape is fabricated as a multi-filament composite superconductor including individual superconducting filaments which extend the length of the multi-filament composite conductor and are surrounded by a matrix-forming material, which is typically silver or another noble metal. Although the matrix forming material conducts electricity, it is not superconducting. Together, the superconducting filaments and the matrix-forming material form the multi-filament composite conductor. In some applications, the superconducting filaments and the matrix-forming material are encased in an insulating layer. The ratio of superconducting material to matrix-forming material is known as the "fill factor" and is generally between 30 and 50%. When the anisotropic superconducting material is formed into a tape, the critical current is often lower when the orientation of an applied magnetic field is perpendicular to the wider surface of the tape, as opposed to when the field is parallel to this wider surface.